Current mirrors are widely used in analog circuit design. Simple current mirrors produce an output current (IO) that is related by a ratio to a reference current (IR). The reference current is received by a reference transistor having an associated width-to-length ratio (αR). The gate of the reference transistor is connected to the gate of a mirror transistor having a gate width-to-length ratio (αR). The magnitude of the reference current IR determines the gate voltage arising at the reference transistor, which is passed to the gate of the mirror transistor. The gate voltage of the mirror transistor determines the magnitude of the output current IO drawn by the mirror transistor.
U.S. Pat. No. 6,462,527 issued to Maneatis discloses one example of a conventional programmable current mirror and includes a reference system 48 having a plurality of transistors 50, 52, 54, 56 coupled in parallel, and a mirror system 49 having a plurality of transistors 58, 60, 62 coupled in parallel as illustrated in FIG. 1. Each of the transistors of the reference system 48 and mirror system 49 has a switch coupled to its source. These current mirrors require different transistor biasing conditions for different control settings resulting in poor performance of the current mirror 49. For example, the biasing voltage at node 100, VGS of transistor 56, for various digital control words are as follows:
Digital Control WordVGS of Transistor 56000      V    GS    =                    2        ⁢                  I          R                            k        ·                  α          R                      +          V      th       001      V    GS    =                    2        ⁢                  I          R                                      k          ·          2                ⁢                  α          R                      +          V      th       010      V    GS    =                    2        ⁢                  I          R                                      k          ·          3                ⁢                  α          R                      +          V      th      
Where,                k=μCOX;        COX is the capacitance of the oxide layer of transistor 56;        Vth is the turn-on voltage of transistor 56; and        μ is the charge carrier effective mobility constant of transistor 56.        
Thus, if the transistor size in the reference system is changed, the biasing voltage will need to be changed.
Accordingly, an improved programmable current mirror is desirable.